JAMB Biology Note- Heredity

Inheritance of Characters in Organisms

Heredity refers to the passing on of traits (characteristics) from parents to offspring through the process of genetic transmission. In all living organisms, the genetic material is carried in genes, which are segments of DNA (deoxyribonucleic acid). These genes determine various characteristics such as height, eye color, and susceptibility to certain diseases.

Table of Contents

• Inheritance of Characters in Organisms
• Heritable and Non-Heritable Characters
• Chromosomes: The Basis of Heredity
  • Structure of Chromosomes
• Transmission of Hereditary Characters: The Process
• Segregation and Recombination of Genes
• Applying Principles of Heredity in Cross-breeding and Selective Breeding
• Genetically Modified Organisms (GMOs) and Gene Therapy
• Advantages and Disadvantages of Inbreeding and Out-breeding
• C) Probability in Genetics and Sex Determination
• A) Application of the Principles of Heredity
  • i) Agriculture
  • ii) Medicine
• B) Sex-Linked Characters
• C) Knowledge of Heredity in Marriage Counseling
• D) Recombinant DNA in Medical Product Production
• E) Identifying Sex-Linked Characters

Heritable and Non-Heritable Characters

• Heritable Characters: These are traits that can be passed from parents to offspring through genes. Examples of heritable characters include:
  • Eye color: Parents pass on genes that control the production of pigments in the eyes.
  • Height: The genes inherited from both parents determine the height of an individual.
  • Blood type: The ABO blood group is inherited according to specific genetic patterns.
  • Disease resistance: Some resistance to diseases may be inherited through specific immune-related genes.
• Non-Heritable Characters: These are traits that cannot be passed from parents to offspring because they are influenced by the environment, lifestyle, or other non-genetic factors. Examples of non-heritable characters include:
  • Scars: Physical marks caused by injury or surgery.
  • Body weight: While genetics may influence body structure, weight can also be influenced by diet, exercise, and lifestyle.
  • Language spoken: This is a learned behavior, not passed down genetically.
  • Tattoos: These are caused by human action and cannot be inherited.

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Chromosomes: The Basis of Heredity

Chromosomes are long strands of DNA that are tightly coiled and stored in the nucleus of cells. They carry the genetic information that is passed from one generation to the next. In humans, there are 46 chromosomes (23 pairs) in every cell (except for reproductive cells, which contain half the number—23 chromosomes).

Each parent contributes one chromosome per pair, and this forms the genetic makeup (genotype) of the offspring. The traits that are inherited from parents are encoded in the sequence of genes located on these chromosomes.

Structure of Chromosomes

A chromosome is made up of:

• DNA: The genetic material that carries instructions for the functioning of cells.
• Histones: Proteins that help in the packaging of DNA, forming a structure known as chromatin.
• Centromere: The central part of a chromosome that helps in its movement during cell division.

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Transmission of Hereditary Characters: The Process

The transmission of hereditary characters follows certain patterns and occurs through genetic inheritance during reproduction.

1. DNA Structure:
   • DNA is a double helix made up of two strands that wind around each other. The strands consist of nucleotides, each containing a sugar molecule, a phosphate group, and a nitrogenous base.
   • The nitrogenous bases (Adenine [A], Thymine [T], Cytosine [C], and Guanine [G]) pair up in a specific way: A pairs with T, and C pairs with G.
2. Meiosis:
   • Meiosis is the process by which gametes (sperm and eggs) are formed. During meiosis, chromosomes undergo segregation, meaning that the two copies of each gene are separated so that each gamete carries only one allele (variant of a gene).
   • The process of crossing over occurs during meiosis, where homologous chromosomes exchange parts, leading to genetic recombination. This increases genetic variation among offspring.
3. Fertilization:
   • During fertilization, the male and female gametes combine to form a zygote, restoring the full number of chromosomes (in humans, 46 chromosomes, 23 from each parent). This recombination of genes is random, meaning that different combinations of genes are passed to offspring with each new generation.

Segregation and Recombination of Genes

• Segregation refers to the separation of alleles during the formation of gametes (sperm and eggs). For example, a parent with a genotype of Aa (heterozygous) will produce two types of gametes: one with allele A and one with allele a.
• Recombination occurs during fertilization, where the genetic material from both parents combines in a new way. This results in offspring with unique genetic combinations different from both parents.

Applying Principles of Heredity in Cross-breeding and Selective Breeding

• Cross-breeding: Cross-breeding involves mating two different individuals with desirable traits to produce offspring that inherit the best characteristics from both parents. For example, cross-breeding plants to produce hybrid crops with improved disease resistance, higher yield, or better taste.
• Inbreeding: This refers to the mating of closely related individuals. While it can help preserve desirable traits within a population, it can also lead to an increase in harmful recessive traits.
• Out-breeding: Out-breeding is the mating of unrelated individuals, which often leads to greater genetic diversity and can reduce the risk of inherited diseases.

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Genetically Modified Organisms (GMOs) and Gene Therapy

• GMOs: Genetically Modified Organisms are organisms whose genetic material has been altered using biotechnology techniques. For example, crops can be engineered to be more resistant to pests, have a longer shelf life, or contain more nutrients. GMOs are controversial, and there are concerns about their impact on biodiversity, the environment, and health.
• Gene Therapy: Gene therapy involves altering the genes inside a person’s cells to treat or prevent disease. This can be done by replacing defective genes, adding new genes, or repairing damaged genes. Although gene therapy has the potential to treat genetic disorders like cystic fibrosis, there are concerns regarding its safety and ethical implications.

Advantages and Disadvantages of Inbreeding and Out-breeding

• Inbreeding:
  • Advantages:
    • Preservation of desirable traits within a population.
    • Homogeneity of certain traits (useful in agriculture and animal breeding).
  • Disadvantages:
    • Increased risk of genetic disorders due to the expression of harmful recessive traits.
    • Reduced genetic diversity, which can make populations more vulnerable to diseases.
• Out-breeding:
  • Advantages:
    • Increased genetic diversity, which can improve the health and adaptability of the population.
    • Reduction in the risk of genetic diseases.
  • Disadvantages:
    • Unpredictability in offspring traits.
    • Potential loss of specific desirable traits over generations.

C) Probability in Genetics and Sex Determination

Probability in genetics refers to the likelihood of inheriting certain traits based on genetic makeup. It is applied using Punnett squares to predict the inheritance patterns of genes. This concept is crucial for understanding the inheritance of dominant and recessive traits, as well as sex-linked traits.

Sex Determination:

• In humans, biological sex is determined by the combination of sex chromosomes: females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
• The Y chromosome carries the SRY gene, which triggers male development.
• When gametes are formed during reproduction, the X and Y chromosomes segregate, and the child inherits one sex chromosome from each parent. If the child inherits an X chromosome from both parents, they will be female (XX), and if they inherit an X chromosome from the mother and a Y chromosome from the father, they will be male (XY).

A) Application of the Principles of Heredity

i) Agriculture

Heredity principles are widely used in agriculture for improving crop and livestock varieties:

• Selective Breeding: Farmers breed animals or plants with desirable traits to produce offspring with improved qualities, such as disease resistance, higher yield, or better nutritional value.
• Genetic Modification: Through genetic engineering, plants can be modified to resist pests, tolerate harsh environmental conditions, or have higher nutritional value (e.g., Golden Rice).

ii) Medicine

The application of heredity principles in medicine has led to advancements in understanding genetic diseases, treatments, and the development of gene therapy:

• Genetic Counseling: Heredity principles are used to predict and diagnose genetic disorders, offering counseling for couples at risk of passing on inherited diseases like cystic fibrosis, sickle-cell anemia, and hemophilia.
• Gene Therapy: Scientists use genetic principles to treat diseases at the genetic level, replacing defective genes with functional ones to cure or alleviate genetic conditions.

B) Sex-Linked Characters

Sex-linked characters are traits controlled by genes located on the sex chromosomes, particularly the X chromosome. They are more common in males due to their single X chromosome.

Examples of Sex-Linked Characters:

• Baldness: Male pattern baldness is a dominant trait controlled by a gene on the X chromosome. Males are more likely to express this trait because they have only one X chromosome.
• Hemophilia: A recessive disorder caused by a defective gene on the X chromosome. It leads to difficulty in blood clotting, and males are more affected as they have only one X chromosome.
• Color Blindness: Also a recessive condition linked to the X chromosome. Males are more frequently affected because they lack a second X chromosome that could compensate for the defective one.

C) Knowledge of Heredity in Marriage Counseling

Marriage counseling can be influenced by an understanding of heredity, particularly concerning the inheritance of blood groups, sickle-cell anemia, and Rhesus factors.

• Blood Grouping: Understanding the parents’ blood groups helps predict the likelihood of their offspring inheriting certain blood types, as well as potential compatibility for blood transfusion.
• Sickle-Cell Anemia: This genetic disorder is inherited in a recessive manner, and counseling can help couples understand the risk of passing the disease to their children, especially if both partners are carriers (heterozygous).
• Rhesus Factor: Couples can be counseled on the importance of Rhesus factor compatibility, particularly in preventing hemolytic disease of the newborn (HDN) if the mother is Rh-negative and the father is Rh-positive.

D) Recombinant DNA in Medical Product Production

Recombinant DNA technology allows scientists to manipulate genes by combining them in new ways to produce medical products. The use of recombinant DNA in medicine has revolutionized the production of important biological products such as:

• Insulin: Recombinant DNA technology enables the production of human insulin by inserting the insulin gene into bacteria, which then produce insulin for use in treating diabetes.
• Interferon: Recombinant DNA is used to produce interferon, a protein that can help treat viral infections and certain types of cancer.
• Enzymes: Recombinant DNA technology is used to produce various enzymes for medical applications, including those used in digestion and blood clotting.

E) Identifying Sex-Linked Characters

Identifying sex-linked characters involves understanding that these traits are located on the X chromosome. To identify them, observe the following:

• Males (XY) will express the trait if they inherit the affected X chromosome, as they only have one X chromosome.
• Females (XX) need two copies of the defective gene (one on each X chromosome) to express the trait. If only one X carries the gene, the female will be a carrier and not express the trait.

Examples of sex-linked traits include color blindness, hemophilia, and Duchenne muscular dystrophy, all of which are more common in males due to their single X chromosome.